In solidity every variable is set to 0 by default. You should think of mappings as all possible combinations are set to 0 by default. In your specific case I would use the following: if (buyers[msg.sender].amount == 0)
Others have already pointed out why it is questionable from a mathematical point of view, by referring to Russell’s paradox. This does not answer your question on a technical level, though. So let’s dissect this: First, once more the relevant part from the JavaDoc of the Set interface: Note: Great care must be exercised if … Read more